How Lost Data Generates Entanglement

The NIST process for using lost data to generate entanglement works like this: Two ultraviolet laser beams entangle the two ion qubits' internal "spins," analogous to tiny bar magnets pointing up or down. The lasers are carefully tuned to couple the ions' synchronized, back-and-forth sideways motion to their spins, entangling this motion with the spins.

The spins have three possible correlations: Both qubits spin up, both spin down, or one is up and one is down. The desired entangled state is a superposition of spins up-down and down-up at the same time. Superposition is another special feature of the quantum world. A measurement of this state with another special-purpose laser beam causes quantum states to collapse, resulting in spins up-down, or the opposite, spins down-up. Such measurements are made by detecting light signals; spin up scatters laser light, whereas spin down does not.

If the two spins are in the desired entangled state and the lowest motional energy state, they are unaffected by all laser and microwave fields. But microwaves and one ultraviolet laser beam reshuffle all other spin states and at the same time boost the qubits to an intermediate state with higher motional energy. This energy is then removed from the qubits by three cooling laser beams applied to the magnesium ions. This continuous feedback loop alters the qubits spins until they settle into the entangled state that is no longer affected by the driving fields.